The laser interferometer is widely used as a length standard due to its ability to provide very high accuracy. For example, almost all CNC and CMM machines in use today in machine shops and factories are routinely calibrated by laser interferometer to ensure accurate performance. Another example is the manufacture of high density semiconductor chips. This is made possible by the laser interferometer, which allows the photomasks for circuitry to be projected on to small areas of wafers. In sum, the laser interferometer plays a crucial role in the advancement of modern technology.
FIG. 1a (background art) is a block diagram showing the basic concepts of linear measurement with a laser interferometer. In this depiction, an outgoing laser beam goes through a beam splitter and is separated into two components. One component, a reference beam, is reflected to a reference cube corner and returns in the opposite direction. The other component, a measurement beam, transmits though the beam splitter and shines on a target cube corner which is mounted on a target object. The measuring beam also returns in the opposite direction, and interferes with the reference beam. Both the reference and measuring beams have an offset caused by the cube corners. The interference signal is at a maximum when these offsets are equal for both beams.
There is no offset if both beams are shining at the apex of cube corners, but the laser beams will then coincide with the outgoing beam from the laser head. There is no place to put the detector in this case and a modification of optical configuration will be needed. For example, quarterwave plates can be placed between the interferometer optics and the target so that the returning laser beams are directed to the direction normal to the original direction. FIG. 1b (background art) is a block diagram showing λ/4 plates added to the interferometer to direct laser beams normal to the direction of the incident beam.
Unfortunately, the laser interferometer has a major limitation in that it can only measure the movement of an object in a straight line. Angular measurement is difficult when a target object moves out of the laser beam path, because the signal is then lost. This is why “angular interferometers” today have a limited measurement range, typically of less than 35 degrees. (Examples of such angular interferometers are provided commercially by Excel Precision Corporation, Agilent Technologies, and Renishaw PLC.)
FIG. 2 (background art) is a block diagram showing what happens when the cube corner on the target object moves in an arc. The cube corner moves out of the laser beam's path and the signal is lost. The displacement of this transverse movement is determined by 2*R*(1-cos θ), where θ is the rotational angle and R is the radius of rotational curvature. The maximum path of the arc at which measurement can continue is at the angle where the reference and measuring beams have smallest overlap but still produce adequate interference.
Theoretically, the closer the cube corner is to the rotation center, the less displacement of offset will occur. The structure of the cube corner, however, has a limitation of its acceptance angle. This full acceptance angle is approximately 40 degrees. So, shortening the radius of the rotation does not bring much advantage. For a 6 mm diameter laser beam, the most ideal radius is approximately 30 mm, and the full acceptance angle is approximately 35 degrees.
Due to the limitation just discussed, the progress of rotational measurement in industry is not as advanced as the progress of linear measurement. Traditional autocollimators are still used in most angular measurement, but these are not only less accurate but also more time consuming. For example, calibration of the X-, Y-, and Z-axes in a manufacturing or laboratory environment typically takes only thirty minutes by laser interferometer, but the rotational measurement of a single axis alone typically takes more than thirty minutes when done by autocollimator.
It therefore follows that the metrology community is badly in need of a laser interferometry technique for rotational measurement, particularly one suitable to measure rotational movement through a complete circle.